Expulsion fuse



Patented Nov. 24, 1931 UNITED STATES PATENT OFFICE JOHN S. NICHOLS, OE rITTSEImD, MASSACHUSETTS, AssIGNoR To GENERAL ELECTRIC COMPANY, A CORPORATION OE NEW YORK EXPULSION FUSE Application led November 18, 1929. Serial No. 407,822.

The present invention relates to electrical protective devices and more especially to expulsion fuses which comprise a fusible element disposed Within an insulating tube or barrel whereby upon fusion or blowing of the element under excess current conditions the gases generated thereby operate to expel the unconsumed parts thereof out the o en end of the tube or barrel and thereby e ect suppression of arcing and the consequent interruption of the flow of electric current.

Among the objects of my invention are: To provide an improved expulsion fuse which shall carry its rated current load indefinitely without vdeterioration or injuryto the tube or barrel or associated parts; to provide an improved expulsion fuse which shall operate effectively and satisfactorily to interrupt the circuit upon the occurrence of excess current conditionsV both on gradual increase above that for which it is rated and on heavy short circuit promptly and reliably; to provide an improved expulsion fuse in which the temperature of the housing shall be maintained relatively low during all normalo Operating conditions and up to the instant of blowing of the fusible element thereof; to provide an improved expulsion fuse which shall be useful in many places and conditions of service which heretofore have necessitated the use of relatively expensive circuit interrupting apparatus, and to provide an expulsion fuse which shall have its point of initial fusion located between the closed end of its housing or barrel and the point of maximum temperature existing in the fusible element.

Further objects will appear and my invention will be better understood from the following description when considered in connection with the accompanying drawings.

An 'embodiment of my invention, with certain modifications, is shown in the accompanying drawings in which Fig. l is a longitudinal sectionof the housing or barrel of an expulsion fuse with the fusible element in operative position therein; Fig. 2 is a bottom end plan thereof; Fig. 3 is a section of a standard cut-out box with the fuse mounted therein, and Figs. l to 7 show in elevation various modifications of the fusible element.

As shown in Figs. l, 2 and 3 of the drawings, the housing or barrel of the expulsion fuse comprises a stout insulating tube l of indurated fibre or similar stout insulating material having mount-ed on its ends metal terminal contacts 2 and The lower terminal contact 2 is located somewhat above the open end of the tube 1 and is provided with a conical surface l and a linurled clamping nut 5 in threaded engagement therewith for electrically connecting or clamping the lower ends 6 of the fusible element thereto, while above the clamping nut 5 it is provided with parallel sided contact. surfaces 7 for engagement with a stationary clip 8 of the cut-out as indicated in Fig. 3.

The upper terminal contact 3 is constructed and mounted on the libre tube 1 so as to form a metallic chambered extension thereof. The lower end of the terminal 3 is provided with parallel sided contact surfaces like 7 of the lower terminal and the upper end 9 is made cylindrical with a thread 10 at the lower portion thereof for engagement with the correspondingly threaded lower end of a clamping thimble l1. The thimble 11 is adapted to engage a soft copper disk 12 of the fusible element and clamp it against the upper end of the Contact 3, andin order to provide a good hand hold thereon -ts outer surface is knurled, as shown in Fig.43. On account of the threaded connection 10 between the thimble 11 and the terminal 3 being removed a considerable distance from the point at which the terminal disk 12 of the fusible elementvis clamped .between them, any metallic gases that may by chance be forced through the joint will come in contact with the extended metal surfaces of the thimble 11 and the outside of the cylindrical part 9 of the terminal 3 and become chilled before they reach the threadedconnection 10 whereby any tendency to sweat together the parts at the threads 10 is avoided.

When the fuse is in operative position in the cut-out box, as indicated in Fig. 3, its contacts 2 and 3 will engage with the stationary clips 8 which are provided with binding posts 13 for engaging the ends of the wires of the circuit served by the cutout. The current entering by the`upper ost 13 will ass by rits clip 8 to the upper use termina 3, through the fusible element whose disk 12 is clamped thereto, and through the lower end strips 6 to the terminal contact 2, to the lower stationafy clip 8 and post 13.

The circuit interrupting element or fuse link is composed of sections or lengths of metal strips 14, 15, 16 and 6. The upper short section 14 is of co per or other suitable metal of relatively igh melting point with one end joined to the terminal disk 12 andthe lower endsoldered to the adjacent section 15. The section 15 is of tin or other -soft metal or alloy having a low melting tion 6 -consists of two sheet cop point and it may be in the form of a sin le strip, as indicated in Figs. 4 to 7, or o a.- plurality, as indicated in Fig. 1. Its length 1s short and when the fusible element is `in operative position in the barrel, the section 15 will be surrounded by the metal inner wall of the upper terminal 3. The long section 16 is of copper or other suitable metalof relatively high melting point and when in operative position in the barrel it is surrounded throughout its length by the fibre tube 1. -It is joined at its upper end to the soft metal section 15 and at -its lower vend to the bifurcated section 6. The bifurcated secr stri s soldered at one end to opposite si es of t e lower end of the long section 16 and when l of the tube 1, they operate substantially to close the lower end thereof and when complete closure is not desired any degree of closure may be obtained by correspondingly varying the width of the strips.

The short copper section 14 which ma. for example, be about 5% inch long definite y locates the low melting point fusible section 15 at a point somewhat below the terminal disc 12 but within the upper metal cavity of the expulsion chamber. This locates the arcing polnt well belowthe terminal disc 12 thereby preventing burning and fusing of the terminal wh1ch might make refusing difficult. It also rovides for the proper control of heat radiation and conduction from the fusible section in order that the advantages of the construction may be realized to the fullest extent. Such spacing of the fusible section from the end of the chamber or cavity also provides an air cushion at the upper end of the fuse holder which acts as a shock absorber and lessens the strain on the tube on heavy short circuits while still affording the proper amount of pressure with a bore of given size to insure the most efficient operation both on overload and on short circuit. f

A wrapper 17 of sheet asbestos about the fusible section 15 serves to protect it during handling and installing and yet is suliiciently thin to have only slight effect upon the convection of heat therefrom. Such wrapping is not essential.

While the long section 16 of the fusible element is shown in Fig. 1, as a flat stri or ribbon, it may be composed of a single raid of line wires, as indicated in Fig. 4, joined at the upper end to a soft metal strip 15. It may be composed'of two smaller wire braids, as shown in Fig. 5, with each braid respectively soldered at its lower end to an end section strip 6while their upper ends are intimately joined to o posite sides of a single soft metal strip 15. n the construction shown in Fig. 6, the section 16 is composed of a rope of fine wires which is divided at the lower end tov form the integral terminal sections 6. It may also be composed of a rope of fine wires, as indicated in Fig. 7, with fiat metal terminal sections 6 solde-red to the lower end, as in the construction of Fig. 1.

i My invention is not limited toany particular metal for the fusible link but it should be a metal having a low melting point. Pure tin, for example, has been found to give very satisfactory results. This fusible link forms a separate and distinct section of the entire fuse rather than a fusible joint or bond between two pieces of metal that are in intimate relation to each other. In the latter case the heat re uired to rupture the fuse is partly due to t e contact between the metal sections whereas in my fusethe'soldered joints may be ample and variations inresistance are avoided which appreciably alect the overload operation of the device. Fuses are common in which there is a restricted section of a relatively high melting point metal, such v as copper, aluminum and the like, but with such metals the heat required to operate a fuse is almost entirely generated in the restricted section and in case of short circuit such a fuse is relatively slow in operation as compared to my fuse. that with such a fuse the temperature, when lis Itis also apparent l the fuse operates, must always be at least equal'to the meltingpoint of the metal which is so high that with the continued passage offa current just alittle below that necessary to blow the fuse the libre tube may be charred. For example', the melting point of copper is 1083o C., while the melting points of aluminum and zinc are respectively 660o C. and 419 `C. On the other hand, the melting kpoint of tin, for example, is only 232 C.

rl`he advantage of my fuse from this standpoint of temperature is illustrated by the fact thatinall expulsion fuses of over 100 amperes rating with which I am rfamiliar the temperature rises as high as 660 C., whereas with my construction no part of the fuse link exceeds a temperature from 231 to 236 C. My fuse, will, therefore, carry its full load current indefinitely without charring the fibre tube, which veiiect takes place with higher yet moderate temperatures which more or less rapidly cause organic insulation material to lose their water of crystallization and carbonize. My fuse avoids this insidious action which, with no external indication to give warning that the fuse is defective, renders the insulating tube conductive so that when an overload or short circuit occurs, and the fusible section ruptures, the current is transferred to the tube which is then not only conductive but in a weakened condition mechanically, and the expulsion effect of the arc beingr absent. the whole cut-out may be destroyed b v a disastrous arc-over between the terminals. Moreover, in my low temperature fuse the amount of heat generated by the fuse link is relatively small due to its peculiar construction and the fuse holder itself with its attached metal partsattains a temperature of only about 125 C. as a maximum.

'Ihe operation of my fuse on overload and on short circuit is distinctly different. Under overload conditions of currents of 110 to 125% ofthe rating of the fuse, the hot spot or region of highest temperature is designed to be approximately in the center of the lower copper section 16. The fusible section is blown by the lheat generated by its resistance and the current flowing through it, augmented by the heat transmitted to it by conduct-ion from the center of the main copper section. Thisgives ay time delay in the operation of the fuse so that the fuse will not blow unnecessarily on overloads of short duration. On short circuits, however, the factors of radiation and conduction are substantially entirely eliminated and the fusible section is volatilized purely by its resistance to the l current. This represents the ideal condition for fuse operation, namely relatively slow operation on low currents, where the circuit shouldnot be opened until the overload is of sutlicient duration to become dangerous, and

extremely rapid operation on shortJ circuit currents where the systemv should be cleared of a fault as quickly-"as possible. ANo' other fuse with which I am familiar possesses these qualities,-those that are fast-'or'qui'ck in operation on short circuitv being proportionately fast on'overload. In my fuse the size of the main copper section is 'carefully designed for the minimum size consistent with proper heating and current carrying capacity and the relative proportions ofthe copper section and the fusible section vare such as to secure the proper functioning'of lthe? fuse in the manner described. Y 1

As indicating the improvement in l'speed of operation of my fuse-referencey may be made, for example, to tests on two 100ampere expulsion fuses both designed to carry 110% continuously and blow at 120 to`150% in've minutes, one fuse being designed in accordance with present standard practice and the other being my low temperature fuse; At 350 amperes my low temperature fuse was found to blow in one second while the' present standard fuse required five and one-halfl seconds. In order to blow thestandard fuse -in one second, 700 amperes were required.

My fuse also has double the-commercial interrupting capacity of any prior expulsion fuse with which I am familiar. My fuse is adapted for commercial operation to interrupt currents of 6000 amperes at 7500 volts at zero power factor, and has successfully interrupted 10,000 amperesr at 5000 volts under the worst power factor conditions. The maximum commercial rating of prior expulsion fuses with which Iam familiar is about 3000 amperes at 7500 volts.

With my v'fuse the pressures developed in the expulsion chamber on short circuits are lower than is the case with prior expulsion fuses with which I am familiar. The very small amount of fusible metal volatilized under short circuit conditions results in a substantial decrease in the pressure developed. This, in turn. brings about higher interrupting capacity due to the larger currents required to produce ressures sufticient to burst the fuse holder. ess shock and mechanical strain are produced on the cover and housing of the cut-out, insuring less breakage on thesel parts under very severe conditions, and less flame and, smoke are ejectced from the cut-out during the interruption of the circuit. An indication of the comparative operation of my fuse with standard expulsion fuses on the market with which I am familiar is given by the following test which was made upon a standard primary cut-out v l n, sizes of fuse links.

perature fuse while in every test but one, fires were started by the standard fuse.

The two thinner copper strips 6 soldered to the lower end of the main section 16 of my fuse secure several important results. They provide a more flexible connection to the clamping ring 5 securing a good contact and low heating from contact resistance. This is particularly important on the larger They provide a fusible joint at the lower lend of the'main section which j oint on overloads is made to fuse and separate by the hot gases from the arcin the upper part of the tube, allowing the main copper strip to be easily ejected and the period of arcing material shortened. The proportions of the fusible section 15, the main section 16 and the lower section 6 are such that the lower fusible joint is fused by the heavy current onshort circuits within a fractional part of a cycle (of a cycle alternating current circuit) after the upper fusible section is ruptured. This again permits the main copper strip to be easily ejected and the arcing time shortened. The double strips, bentin opposite ways, constitute a barrier or pressure obstruction to the emission of. hot gases due to the delay in fusing of the lower joint by the hot gases on overload, and the purpose is served of building up the pressure in the tube to a point where the overload operation is made more reliable than when the gases are allowed to escape freely. The much quicker fusing of this lower joint on short circuit permits the removal of the obstruction more quickly so that pressures are not excessive or beyond the ability of the fuse holder to handle.

The advantages of the construction of the entire fuse are such that I have been able, for example, to produce a fuse. which will carry 225 amperes that is only approximately onehalf the physical size required for pressent fuses with which I am familiar. This makes possible an increase in the rating of standard cut-outs without anv increasein their size.

Vhile I prefer to use the lower heat disruptable jo1nt for'the reasons heretofore set forth, it is apparent that some of the advantages of my invention may be obtained with a construction such as shown in Fig. 6 wherein the lower bifurcations of the stranded main section are acted upon by the gasesto pull the flexible main section down and eject it from the lower open end of the tube.

While in some of the appended claims I refer to copper and tin for simplicity of expression it will be understood that equivalent metals or alloys may be used, and it will be clear to those skilled in the art from the foregoing specification what characteristics the metals or alloys should have to be equivalents.

While in the foregoing specification, I have .described certain embodiments of my invention and have given various statements of operating capacity by way of example, it w1ll be apparent to those skilled in the art that changes may be made without departin from my invention and I, therefore, aim in t e appended claims to cover all such changes as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is,-

1. An expulsion fuse comprising a barrel open at one end and closed at the other vend and consistsing ofa tube of insulation having attached at one end a metallic terminal having a chamber forming an extension of the bore of the tube and a terminal adjacent the other end, a circuit interrupting element within said barrel and comprlsing a section of low melting point metal disposed within and connectedto said closed terminal, a long section of hard metal .connected at one end to said soft metal section'and centrally disposed within said insulating tube, and a bifurcated section of flexible metal connected within said tube to the other end of said long section by a heat disruptable joint and extended in opposite directions partially to close the open end oa said tube and connected outside thereof to said open terminal.

2. A fuse link comprising a short.V section of hard metal, a short section of low melting point metal join d at one end to said har metal section, a ong" metal section of hard metal joined at one end to the other end of said soft metal section and a bifurcated sec,- tion connected to the other end of said hard metal section by a heat disruptable joint.

3. A fuse link comprising a short section of copper, a short section of tin connected at one end to said copper section, a long copper section connected at one end to the other end of said tin section; and a pair of flexible sheet copper strips soldered to opposite sides of the other end of said long copper section.

4. An expulsion fuse comprising a tube ofV insulating material with a terminal member adjacent each end, means forming a metal' chamber closing the upper end of said tube and in electrical connection with one fuse terminal, the lower end of said tube being open, a fuse link located substantially axially of said tube and chamber and comprising a relatively` short fusible section of tin, a relatively short section of co per electrically connecting one end of sai tin section to said metal chamber and securing said tin section within said metal chamber a short distance from the closed end thereof to provide a compression chamber above the tin section, a relatively long copper section joined to the other end of said tin section and extending to a point near the lower open end of said tube, and conducting means soldered to the lower end of said long copper section extending to the other fuse terminal and acting partially to obstruct the initial escape of gases upon the blowin of the tin section, said soldered connection eing adapted to be unsoldered by the -hot gases an instant after the tin section fuses, said fuse link being proportioned so that the region of highest temperature is in the lower copper sectlon and appreciably below the tin section.

In witness whereof, I have hereunto set, my handthis 15th day of Nov., 1929.

- JOHN S. NICHOLS.

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